False-Colour Photography: a Novel Digital Approach to Visualize the Bee View of Flowers

Journal of Pollination Ecology, 23(12), 2018, pp 102-118 FALSE-COLOUR PHOTOGRAPHY: A NOVEL DIGITAL APPROACH TO VISUALIZE THE BEE VIEW OF FLOWERS Christian Verhoeven1, Zong-Xin Ren2, and Klaus Lunau1 1Institute of Sensory Ecology, Department Biology, Heinrich-Heine-University Düsseldorf, Universitätsstr. 1, 40225 Düsseldorf, Germany 2Key Laboratory for Plant Diversity and Biogeography of East Asia, Kunming Institute of Botany, Chinese Academy of Sciences, Kunming, Yunnan 650201, People’s Republic of China Abstract—The colour vision system of bees and humans differs mainly in that, contrary to humans, bees are sensitive to ultraviolet light and insensitive to red light. The synopsis of a colour picture and a UV picture is inappropriate to illustrate the bee view of flowers, since the colour picture does not exclude red light. In this study false-colour pictures in bee view are assembled from digital photos taken through a UV, a blue, and a green filter matching the spectral sensitivity of the bees’ photoreceptors. False-colour pictures demonstrate small-sized colour patterns in flowers, e.g. based on pollen grains, anthers, filamental hairs, and other tiny structures that are inaccessible to spectrophotometry. Moreover, false-colour pictures are suited to demonstrate flowers and floral parts that are conspicuous or inconspicuous to bees. False-colour pictures also direct the attention to other ranges of wavelength besides ultraviolet demonstrating for example blue and yellow bulls’ eyes in addition to UV bulls’ eyes which previously have been overlooked. False-colour photography is a robust method that can be used under field conditions, with various equipment and with simple colour editing. Keywords: bee, colour vision, flower colour, colour pattern, bee view, false colour INTRODUCTION Flowers serve as filters signalling attractants for pollinators as well as deterrents for flower antagonists (Junker & Blüthgen 2010). Due to differences in the number of photoreceptors types, the spectral range of sensitivity of the colour photoreceptors, and differences in the colour preference of various flower visitors, flowers can be attractive to pollinators and unattractive to illegitimate visitors, i.e. pollen thieves and nectar robbers, at the same time (Lunau & Maier 1995; Lunau et al. 2011). Since many flower visitors, such as hummingbirds (Herrera et al. 2008), bats (Winter et al. 2003), bees (Peitsch et al. 1992), hoverflies (Lunau 2014), beetles (Martínez-Harms et al. 2012), butterflies and moths (Kelber et al. 2002; Arikawa 2003) are sensitive to ultraviolet light, these flower visitors necessarily view different aspects of the spectral reflectance of flowers as compared to UV-blind humans. Many of these flower visitors possess a different number of colour photoreceptor types compared to humans, FIGURE 1. Spectral sensitivity of colour photoreceptors. Top: e.g. one in bats, two in beetles, three in bees, four in Relative sensitivity of the three human photoreceptor types. The hummingbirds and flies, and from three up to fifteen in curves peak at 420 nm, 534 nm and 564 nm (Bowmaker et al. 1980). butterflies (Briscoe 2008; Chen et al. 2016). This results in In the UV range of wavelength between 300 nm and 400 nm the colour vision systems hardly conceivable to that in humans. photoreceptors are not sensitive (modified from Stockman et al. However, bees have trichromatic colour vision just like 1993). Bottom: Relative sensitivity of the three photoreceptor types humans, but are sensitive to ultraviolet, blue and green light of the honeybee (Apis mellifera). The curves peak at 344 nm, 436 rather than blue, green and red light as humans (Peitsch et al. nm and 544 nm. In the red range of wavelength between 630 nm and 1992; Fig. 1). Neuronal processing of colour photoreceptor 700 nm (red light) the photoreceptors are not sensitive (modified from Peitsch et al. 1992). inputs in bees and humans is similar, both bees and humans Received 22 January 2018, accepted 22 May 2018 have colour opponent neurons (Backhaus 1991) and colour *Corresponding author: [email protected] constancy (Neumeyer 1981). To understand flower colours and floral colours pattern as bees might see them Kevan 102 June 2018 FALSE-COLOUR PHOTOGRAPHY IN BEE VIEW 103 (1978, 1979, 1983) claimed the need for a bee-subjective made by a UV-sensitive camera and specially selected set of colorimetric analysis of flower colours. filters to reconstruct images of flowers as bees might see them by calculating the quantum catches of the honeybee Three methods are frequently used to study floral colours photoreceptors for each ommatidium dependent on the and colour patterns as perceived by bees. The measurement of distance between bee and flower. Multispectral images, a series the spectral reflectance by a spectrophotometer allows for the of black-and-white photos taken through different bandpass comparison of the spectral reflectance of a floral tissue with filters (Kevan 1972; Chittka and Kevan 2005) from alpine that of a black standard and of a white standard. A database flowers of the Rocky Mountains, Colorado, are available from provides access to spectral reflectance measurements for many the Floral Image Database (http://liu.edu/flower). Benitez- plant species (Arnold et al. 2010). Reflection spectroscopy is Vieyra et al. (2007), Williams & Dyer (2007), Brito et al. limited by the size and surface properties of the measuring (2015) and Hempel de Ibarra et al. (2015) presented false spot, which does not allow for measuring the reflectance of colour photos of flowers in bee view, some of which require small-sized, curved or rough-textured structures like pollen sophisticated calculations or calibrations. Photographic grains, stamens, and filamental hairs. The standard method of methods have also been used for simultaneous quantitative reflection spectroscopy is a measurement angle of 45° without measurements of spatial and spectral components of floral integrating sphere (Chittka & Kevan 2005; but see Stavenga colour patterns as seen by bees (Vorobyev et al. 1997; Benitez- & van der Kooi 2016). This method largely ignores the impact Vieyra et al. 2007; Williams & Dyer 2007; Hempel de Ibarra of the surface of petals being flat, conical or other, since most & Vorobyev 2009; Garcia et al. 2014; Hempel de Ibarra et al. white standards possess a smooth if not glossy surface. 2015; White et al. 2015). Moreover, the transparency of the petals is ignored, since the flower, e.g. a petal, is placed on different backgrounds being Recent studies presenting multispectral images from black, white or another petal for measurements, which flowers in bee view (Garcia et al. 2014; Brito et al. 2015; disregards light shining through the petal from the backside Hempel de Ibarra et al. 2015; White et al. 2015; Vasas et al. and the impact of the colour chosen as background. 2017) are mostly focusing on one or few target species and /or require sophisticated calculations or calibrations. In this Another method is the synopsis of a colour picture and a study we introduce a simple and cheap method for general use UV-picture showing the flower as appearing to humans and based on a slightly modified and supplemented standard the ultraviolet reflection pattern on different pictures camera equipment to create UV-pictures and false-colour (Daumer 1958; Silberglied 1979). This method possesses the pictures in bee view and to highlight the rationale of this disadvantage that the readers might be successful in notionally method for field studies. The estimated prices for a low-cost superimposing the colour picture and the UV-picture but fail and a standard equipment as used in this study are shown in to eliminate the red content of the colour picture at the same Supplement S1. This equipment is much less expensive than time. Red, however, is a component of yellow, magenta, purple custom-built multispectral cameras, e.g. Spectrocam and white colours. Moreover, the interpretation of UV- Multispectral Wheel Cameras (Ocean Optics). The pictures pictures showing UV-absorbing parts of flowers is sometimes are intended to demonstrate the bee-subjective colour patterns misleading, e.g. if it is concluded that red and UV-absorbing of flowers in bee view with details that cannot be measured parts are invisible to bees (Chittka & Waser 1997). A using a normal spectrophotometer. Moreover, it is television camera equipped with UV-transmitting lens and demonstrated how false-colour pictures in bee view are used UV-transmitting filter allowed direct observation and to better understand the impact of the bees’ sensitivity for recording on video tape (Eisner et al. 1969; Silberglied 1976; ultraviolet and insensitivity for red light. For this purpose, Gronquist et al. 2001). Also, a modified image converter and false-colour pictures of floral colour changes and small-sized image amplifier combined with UV-transmitting filter and floral structures are shown as false-colour pictures. In UV-transmitting lens enabled to screen the UV-reflectance addition, yet undescribed forms of colour patterns are properties of many flowers (Rosen & Barthlott 1991) in the described using false colour pictures such as blue bull’s eyes field. However, classical UV-photography using a UV- rather than ultraviolet bull’s eyes (Silberglied 1979; Koski & sensitive camera mounted on a tripod, UV-transmitting filter Ashman 2014). The robustness of this method is tested by and UV-transmitting lens and a UV-sensitive film were still the comparison of false-colour pictures taken with different preferred to produce UV-photos with high quality (Biedinger cameras, lenses, filters, different picture editing programs and & Barthlott 1993; Burr & Barthlott 1993; Burr et al. 1995; in field conditions. Lunau 2000). With the turn of the millennium, digital cameras were used mostly after having their internal UV- and MATERIALS AND METHODS IR-blocking filter removed and sometimes with the support of a UV-lamp for illumination (Koski & Ashman 2013). False-colour photography of flowers and floral colour Several attempts have been made to develop methods to patterns in bee view is an extension to the well-established create colour pictures in bee view.

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